The escalating requirements for high density and performance associated with ultra large scale integration semiconductor wiring require responsive changes in interconnection technology, which is considered one of the most demanding aspects of ultra large scale integration technology. Such escalating requirements have been found difficult to satisfy in terms of providing a low RC (resistance capacitance) interconnect pattern, particularly wherein submicron vias, contacts and trenches have high aspect ratios due to miniaturization.
Conventional semiconductor devices comprise a semiconductor substrate, normally of monocrystalline silicon, and a plurality of sequentially formed dielectric interlayers and conductive patterns. An integrated circuit is formed containing a plurality of conductive patterns comprising conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the conductive patterns on different layers, i.e., upper and lower layers, are electrically connected by a conductive plug filling a via opening, while a conductive plug filling a contact opening establishes electrical contact with an active region on a semiconductor substrate, such as a source/drain region. Conductive lines formed in trench openings typically extend substantially horizontal with respect to the semiconductor substrate. Semiconductor "chips" comprising four or more levels of metallization are becoming more prevalent as device geometries shrink to submicron levels.
A conductive plug filling a via opening is typically formed by depositing a dielectric interlayer on a conductive layer comprising at least one conductive pattern, forming an opening through the dielectric interlayer by conventional photolithographic and etching techniques, and filling the opening with a conductive material, such as tungsten (W). Excess conductive material on the surface of the dielectric interlayer can be removed by chemical-mechanical polishing. One such method is known as damascene and basically involves the formation of an opening which is filled in with a metal. Dual damascene techniques involve the formation of an opening comprising a lower contact or via opening section in communication with an upper trench opening section, which opening is filled with a conductive material, typically a metal, to simultaneously form a conductive plug in electrical contact with a conductive line. In copending application Ser. No. 08/320,516 filed on Oct. 11, 1994, now U.S. Pat. No. 5,635,423, prior art single and dual damascene techniques are disclosed, in addition to several improved dual damascene techniques simultaneously forming a conductive line in electrical contact with a conductive plug for greater accuracy in forming fine line patterns with minimal interwiring spacings.
High performance microprocessor applications require rapid speed of semiconductor circuitry. The control speed of semiconductor circuitry varies inversely with the resistance and capacitance of the interconnection pattern. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. Thus, the interconnection pattern limits the speed of the integrated circuit.
If the interconnection node is routed over a considerable distance, e.g., hundreds of microns or more, as in submicron technologies, the interconnection capacitance limits the circuit node capacitance loading and, hence, the circuit speed. As integration density increases and feature size decreases in accordance with submicron design rules, the rejection rate due to integrated circuit speed delays approaches and even exceeds 20%.
One way to increase the control speed of semiconductor circuitry is to reduce the resistance of a conductive pattern. Conventional metallization patterns are typically formed by depositing a layer of conductive material, notably aluminum (Al) or an alloy thereof, and etching, or by damascene techniques wherein trenches are formed in dielectric layers and filled with a conductive material. Excess conductive material on the surface of the dielectric layer is then removed by chemical-mechanical polishing. Al is conventionally employed because it is relatively inexpensive, exhibits low resistivity and is relatively easy to etch. However, as the size of openings for vias/contacts and trenches is scaled down to the sub-micron range, step coverage problems have arisen involving the use of Al which has decreased the reliability of interconnections formed between different wiring layers. Such poor step coverage results in high current density and enhanced electromigration. Moreover, low dielectric constant polyimide materials, when employed as dielectric interlayers, create moisture/bias reliability problems when in contact with Al.
One approach to improved interconnection paths in vias comprises the use of completely filled plugs of a metal, such as W. Accordingly, many current semiconductor devices utilizing VLSI (very large scale integration) technology employ Al for a wiring metal and W plugs for interconnections at different levels. However, the use W is attendant with several disadvantages. For example, most W processes are complex and expensive. Moreover, W has a high resistivity. The Joule heating may enhance electromigration of adjacent Al wiring. Furthermore, W plugs are susceptible to void formation and the interface with the wiring layer usually results in high contact resistance.
Another attempted solution for the Al plug interconnect problem comprises the use of chemical vapor deposition (CVD) or physical vapor deposition (PVD) at elevated temperatures for Al deposition. The use of CVD for depositing Al has proven expensive, while hot PVD Al deposition requires very high process temperatures incompatible with manufacturing integrated circuitry.
Copper (Cu) and Cu alloys have recently received considerable attention as a replacement material for Al in VLSI interconnect metallizations. Cu exhibits superior electromigration properties and has a lower resistivity than Al. In addition, Cu has improved electrical properties vis-a-vis W, making Cu a desirable metal for use as a conductive plug as well as conductive wiring. However, there are also disadvantages attendant upon the use of Cu. For example, Cu metallization is very difficult to etch. Moreover, Cu readily diffuses through silicon dioxide, the typical dielectric interlayer material employed in the manufacture of semiconductor devices, and adversely affects the devices.
One conventional approach in attempting to form Cu plugs and wiring comprises the use of damascene structures employing chemical mechanical polishing, as in Chow et al., U.S. Pat. No. 4,789,648. However, due to Cu diffusion through dielectric interlayer materials, such as silicon dioxide, Cu interconnect structures must be encapsulated by a diffusion barrier layer. Typical diffusion barrier metals include tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN), titanium tungsten (TiW), and silicon nitride (Si.sub.3 N.sub.4) for encapsulating Cu. The use of such barrier materials to encapsulate Cu is not limited to the interface between Cu and the dielectric interlayer, but includes interfaces with other metals as well.
Electroless deposition has been suggested as a technique for forming interconnect structures. See, for example, "Electroless Cu for VLSI," Cho et al., MRS Bulletin, June 1993, pp. 31-38; "Selective Electroless Metal Deposition For Integrated Circuit Fabrication," Ting et al., J. Electrochem. Soc., 136, 1989, p. 456 et seq.; "Selective Electroless Metal Deposition For Via Hole Filling in VLSI Multilevel Interconnection Structures," Ting et al., J. Electrochem. Soc., 136, 1989, p. 462 et seq.; and Shacham et al., U.S. Pat. No. 5,240,497.
Electroless Cu deposition is attractive due to low processing costs and high quality Cu deposits. In addition, equipment for performing electroless metal deposition is relatively inexpensive vis-a-vis other semiconductor processing equipment for depositing metals. Electroless deposition also offers the advantageous opportunity for batch processing of wafers, thereby further reducing the cost of electroless deposition and increasing production throughput. However, electroless deposition requires a catalytic surface, i.e., seed layer, for the autocatalytic action to occur. See, for example, Baum et al., U.S. Pat. No. 4,574,095 and "Electroless Copper Deposition on Metals and Silicides," Mak, MRS Bulletin, August 1994, pp. 55-62. It is difficult to obtain reliable and reproducible electroless Cu deposition, since the seed layer surface must maintain catalytic activity for effective electroless deposition of Cu.
Copending application Ser. No. 08/587,264, filed Jan. 16, 1996, now U.S. Pat. No. 5,824,599, discloses a method of electrolessly depositing Cu in an interconnect structure, which method comprises initially depositing a barrier layer in an opening, depositing a catalytic seed layer, preferably of Cu, on the barrier layer, and then depositing a protective layer the catalytic layer encapsulating and protecting the catalytic layer from oxidation. The preferred protective material is Al which forms an Al-Cu alloy at the interface of the catalytic and protective layers, thereby encapsulating the underlying Cu. Subsequently, Cu is electrolessly deposited from an electroless deposition solution which dissolves the overlying protective alloy layer to expose the underlying catalytic Cu layer.
As the aspect ratio of openings for trenches, contacts and vias for interconnect patterns approaches 2:1 and greater, e.g., greater than 3:1, it becomes increasingly more challenging to voidlessly fill such openings employing conventional technology, such as magnetron sputtering techniques involving either direct current or radio frequency sputtering. Conventional attempts to improve sputtering capabilities comprise the use of a collimator as in Sandhu et al., U.S. Pat. No. 5,409,587.
A more recent approach in the evolution of high aspect ratio contact/via interconnection technology involves the ionization of sputtered metals by a high density plasma. See S. M. Rossnagel et al., "Metal ion deposition from ionized magnetron sputtering discharge,"J. Vac. Sci. Technol. B 12(1), January/Febuary 1994, pp. 449-453 and J. Hopwood et al., "Mechanisms for highly ionized magnetron sputtering," J. Appl. Phys., Vol. 78, No. 2, Jul. 15, 1995, pp. 758-765. Further attempts to improve RF induced plasma processing by generating a greater percent of ionized sputtered material employing a coil having a generally flattened surface defined by parallel conductors is disclosed by Cuomo et al., U.S. Pat. No. 5,280,154.
Although electroless deposition and electroplating offer the prospect of low cost, high throughput, high quality plated films and efficient via, contact and trench filling capabilities, the requirement for a catalytic seed layer becomes problematic, particularly in filling high aspect ratio openings. Electroless plating generally involves the controlled autocatalytic deposition of a continuous film on the catalytic surface by the interaction in solution of a metal salt and a chemical reducing agent. Electroplating comprises the electrodeposition of an adherent metallic coating on an electrode employing externally supplied electrons to reduce metal ions in the plating solution. A seed layer is required to catalyze electroless deposition or to carry electrical current for electroplating. For electroplating, the seed layer must be continuous. However, for electroless plating, very thin catalytic layers, e.g., less than 100 .ANG., can be employed in the form of islets of catalytic metal.
In copending application Ser. No. 08/857,129 filed on May 15, 1997 (Our Docket No. 1033-226/150100-301, a method of electroless plating or electroplating copper or a copper alloy to fill high aspect ratio openings is disclosed, wherein a seed layer comprising an alloy of a refractory metal and one or more additive metals is initially deposited.
There is a continuing need for improvements in electroplating Cu or Cu alloys, particularly in voidlessly filling high aspect ratio openings, e.g., aspect ratios greater than 3:1 and with improved uniformity. There is also a need to develop methods for electroplating copper and copper alloys to voidlessly fill high aspect ratio openings so that the deposited copper or copper alloy exhibits a large grain size for improved reliability and increased electromigration resistance.